This invention relates to fuel injection systems for internal combustion engines, and more particularly to improvements therein.
In the operation of medium and high-speed compression-ignition engines, a compromise is always made between economy, smoke, peak cylinder pressure and more recently, emissions. The fuel injection system appears to be the factor which has the most significant influence on these variables, since limitations in its capabilities set practical boundaries to the choice of operational parameters.
It has been demonstrated that both speed and load influence ideal injection timing. Of the few conventional injection systems that provide timing adjustment, speed control advance is more commonly chosen. A few systems provide speed and load advance, but the mechanism required is heavy, complex and expensive. When unit pump/injectors--driven by a cam, push rod and rocker from the engine camshaft--are employed, it is not practical to have controlled timing adjustment. Uncontrolled timing variations occur in some of these systems as the fueling is altered.
Control of the combustion by injection timing and/or rate is always attempted in engine development. For example, injection timing is adjusted to hold peak cylinder pressure within design limits. However, the employment of a programmed, non-uniform injection rate can be shown to reduce peak cylinder pressure without significantly influencing economy. A complete investigation of this mode of combustion control has been prevented because conventional injection systems lack the flexibility to achieve rapid changes in injection rate. Furthermore, since they derive injection energy directly from the crankshaft at the time injection is taking place, the entire energy transfer takes place over a small crankshaft angle and very high forces or torques are involved. Consequently, mechanical drives must be rugged and the requirement for variable timing becomes more difficult to satisfy. Torsional impulses are returned to the crankshaft, increasing engine roughness and placing greater demands upon the torsional vibration damper.
Fuels of varying cetane value and specific gravity generally require different injection timings for optimum combustion. The extent of the timing change required for a range of fuels is greatly dependent upon the engine's design, but frequently the injection timing cannot be maintained at optimum by a single step adjustment. Existing injection systems can be provided with several alternative timing points. Changes in fuel type can be accommodated by manually selecting the appropriate setting of the pump coupling to give a compromise timing curve. However, at some speeds and and loads, losses in combustion efficiency occur.
Mechanically controlled injection introduces a programmed maximum fuel vs. speed characteristic by the complicated interaction of dynamic hydraulic effects. Adjustments of the fuel pipe unloading volume, pipe size, nozzle characteristics, pump element diameter, and pump cam are made during development to achieve the torque shaping desired for the particular vehicle or application. Subsequent change in the desired torque curve requires partial or complete rebuild of the fuel injection equipment. Similarly, with speed-governing, changes in the governed speed, tolerable "runout" or "all speed" characteristics demand a mechanical rebuild.
This lack of flexibility in the control of mechanical systems requires a large inventory of spare parts or replacement fuel injection equipment sets and tends to prevent the adaptability of complete rebuilt engines to a variety of vehicle installations.